In an apparatus for forming a color image, such as a color printer and a color copying machine, a color image on an original is optically separated into different color signals, the separated color signals are converted into print signals (e.g., toner signals) for coloring materials, such as color toners, color ink, and ink toner film, by a color masking process, and the print signals are supplied to a printer section. The separated color signals of blue (B), green (G) and red (R) are converted into print signals for coloring materials, i.e., color signals of yellow (Y), magenta (M) and cyan (C), in a color masking process by the following matrix operation ##EQU1##
Through the above conversion process, color tone and chroma of a reproduced image is controlled while keeping a good color balance.
The color signals are frequently affected by the spectrum of an illumination light source, characteristics of a dichroic mirror, further color characteristics of the photo sensors, and color filters, lenses, and the like. Also, in the printer section, the coloring materials used for printing the color image have unnecessary absorptions in their spectral characteristics. As a result, hue and chroma in the reproduced image become different from hue and chroma in the original image. The image quality of the reproduced image is degraded and many corrections are needed in the printer section. Many additional factors exist that adversely affect the tone reproduction, such as characteristics of the paper and the ink color. As seen from the above description, an exact color reproduction of the original color is almost impossible by mere application of simple color conversions to the color image signals.
In color image processing, to obtain an exact reproduction of the original color, the color masking process follows various processes, for example, an equivalent neutral density (END) conversion, tone reproduction control (TRC), and others.
Generally, to ease processing, the B, G, and R signals and the Y, M, and C signals are frequently expressed in terms of the END. In this case, where gray is read the separated color signals B, G, and R are equal, i.e., B=G=R (equivalent neutral density). When the signals Y, M, and C are equal, i.e., when the values Y=M=C are sent to the printer section, a gray image is printed.
To be more specific, a color image of an original is read in terms of a trio of color signals B, G, and R, as described above. When a gray (achromatic) image is read, the separated color signals G, B, and R are also obtained, but these signals are not actually equal because the characteristics and operating conditions of the image reader are not uniform. For this reason, the END conversion is used.
The END conversion controls the image signals B, G, and R to be in the same gray density. The separated color signals B, G, and R are converted into print signals Y, M, and C for the coloring materials, such as toner and ink. A black print signal K (black or tusche) is formed by using the signals Y, M, and C. The desired equal value for the signals Y, M, and C, which depend on the value of the generated K, are subtracted from the original values of the signals Y, M, and C. These signals are sent to the printer section. Finally, the printer section reproduces the color image of the original.
If the gray signal that results from the equal coloring material signals, Y=M=C, is applied to the printer section, the printer section cannot exactly reproduce the gray of the original image, because optical characteristics of the coloring materials and operating conditions of the printer section affect the signals. However, when the signals Y, M, and C are equal, a gray obtained under such a condition will exactly be reproduced.
The color masking process is for converting the separated color signals B, G, and R into the coloring material signals Y, M, and C. In this process, it is an important matter how to exactly reproduce colors other than gray, and to reproduce hue, saturation, and the like of the other colors. Thus, the END conversion, color masking, and TRC (tone reproduction control) play important roles in the improvement of the reproduction of colors including gray.
As described above, the conventional color image forming apparatus controls the color correction/conversion and the tone reproduction while maintaining the gray balance and gradation by END conversion, to reproduce an image excellent in tone, definition and graininess. Thus, the conventional color image forming apparatus corrects colors on the basis of gray. However, the gray balance conditions are greatly influenced by the conversions of image data, edit, input conditions of originals, printing conditions, and the like. Therefore, it is difficult to maintain a good gray balance. If the gray balance is lost, it is difficult to reproduce a color image whose color is exactly the same as that of the original one.
To obtain a good reproduction of an original image, such as by photo, characters, mesh point, and photo/character, and to reproduce a color image by a film projector, the gray balance must be harmonized with the color change, and the color image forming apparatus must be arranged so as to accept the above requirements flexibly. A color image forming apparatus with the above two functions, when realized by the conventional technique, must use a large capacity memory. Accordingly, the data processing speed of the apparatus is slow. To improve the data processing speed, if the memory capacity is reduced, the image reproduction becomes poor.